The Coanda effect is a phenomenon which tends to keep a jet of fluid attached to a surface over which it flows. It is discussed in a paper by Gregory-Smith entitled “The Discharge from a thin slot over a surface of convex curvature” (Int. J. Mech. Sci. Vol 24 No. 6 pp 329-339). This paper reports on an experimental study to determine the minimum radius r which the jet will follow without breaking away from it.
The results of the above experiments show that for any given P0/Pa ratio (where P0 is the total pressure and Pa is the ambient pressure) there is a value of b/r (where b is the jet width) below which the jet will be attached to the curved surface.
Above this value there is a range of b/r values where the jet is bistable in the sense that, on start-up, the jet will separate from the curved surface but, if constrained to follow it by some external effect, will then remain attached.
Above another value, the jet will separate from the curved surface and the Coanda effect does not exist.
Existing literature includes many fanciful descriptions of flying machines in the shape of inverted “saucers.” For example, Patent Specification GB2387158 describes a proposal where a fan directs air over a convex disc to produce lift. Patent Specification U.S. Pat. Nos. 5,503,351 and 3,276,723 describe arrangements where an air jet flows on opposite sides of a disc shaped aerofoil to create lift. U.S. Pat. No. 5,803,199 describes a hovercraft that also uses airflow over an outside surface of the craft to achieve a supplementary lifting effect. U.S. Pat. No. 5,054,713 describes an arrangement in which an air jet flows over an “oblately spheroidal” body to derive lift. Each of these known proposals either fails to discuss the precise curvature of the aerofoil surface or assumes that conventional practices associated with jet flow over a surface curved in a single plane will equally apply for surfaces that have double convex curvature. Patent specification U.S. Pat. No. 2,978,206 describes a vehicle where a fan causes air to flow over a surface that is described as generally parabolic and that has a tight radius of curvature at the downstream edge of the surface.